More particularly, the invention relates to an acousto-optic imaging method.
In this type of method, an object to be imaged is illuminated with a laser-type light source. Moreover, acoustic waves are propagated in the object by an ultrasound source. Information is obtained for an area of the object to be imaged by detecting a signal linked to the coupling properties between the light wave and the ultrasound wave which specifically makes the concerned area vibrate. In practice, when an ultrasound wave, of acoustic frequency fa passes through a scattering medium (such as, for example, a biological tissue, or other), it provokes a periodic shaking of the scatterers and a periodic modulation of the refraction index of the medium. If a laser wave, of incident frequency fI, is scattered by the medium, the movement of the scatterers and the modulation of the index of the medium generate a signal wave comprising on the one hand a carrier component (at the frequency fI) and on the other hand, an acousto-optic component scattered on one or other of the acoustic side-bands (of frequency fAO=fa±fI). Acousto-optic imaging consists in determining the weight of this component at the frequency fAO according to the focal position of the acoustic wave in the diffusing medium.
Historically, detection was initially performed using a single-pixel detector. However, this technique offers poor sensibility.
In practice, the detection is achieved by measuring the interferences between two components of the signal wave: the carrier component, at the frequency fI, and the acousto-optic component, at the frequency fAO. Since these two frequencies differ from each other roughly by the value of the acoustic frequency fa of the ultrasound wave, the detection is heterodyne. Such a detection is effective only for a very small geometric expanse, such that most of the signal is lost.
Also, because of the presence of diffusers in the medium, the carrier and acousto-optic components of the signal wave are two random speckle fields, such that the relevant information is obtained only by spatially and/or temporally averaging the detected signal.
A major improvement was provided by the ESPCI (see in particular “Ultrasonic tagging of photon paths in scattering media: parallel speckle modulation”, Levêque et al., published in Optic Letters 24: 181, 1999). In this device, the single-pixel detector is replaced by a multiple-pixel detector such as a CCD camera. There is, however, a problem in that such a camera is too slow to detect an interference signal between the carrier and acousto-optic components of the signal wave, which has a high frequency, of the order of that of the acoustic wave (a few MHz, typically). To detect a signal, the ESPCI no longer detects the interferences of the acousto-optic component with the carrier component, but with a reference component passing through the medium and obtained by amplitude modulation of the incident wave at a frequency close to that of the acousto-optic component (typically, to within a few Hz). There is thus obtained an interference between the reference component and the acousto-optic component which is slow enough to be detected for each pixel of the camera. To obtain the information relating to the vibrating area, the detected signal must be summed over all the pixels of the camera.
This technique is not, however, the best possible because, on the one hand, the measured signal includes a significant noise component due to the photons that are simply scattered having passed through an area of the object to be imaged that is not vibrating, and on the other hand, the reference component is relatively weak, because it passes through the scattering medium.
Furthermore, each of the signals carries a so-called “speckle decorrelation” noise. The light, scattered by the medium, is emitted in the form of a speckle wave, made up of grains. From one speckle grain to the next, the amplitude and the phase of the signal wave vary randomly. If, over time, the scattering medium is modified (such is in particular the case for living tissues), the scatterers change position. This modifies the position, the intensity and the phase of the speckle grains (the speckle is said to be decorrelated).
In acousto-optic imaging, the overall intensity of the acousto-optic component of the signal wave is to be measured. The latter is much weaker than that corresponding to the acoustically unmarked component (the carrier component or the reference component, depending on the used technique), which is also seen by the detector. If a continuous part associated with the acoustically unmarked component can easily be eliminated during the detection, the amplitude and phase variations of the signal wave, which are reflected by the decorrelation of the speckle, often lead to a false signal called “speckle decorrelation noise”.
Living biological tissues, for which the acousto-optic imaging technique is required to be used, for example in screening for breast cancer, or other purposes, lead to a major speckle decorrelation noise. It is therefore preferable to be able to perform rapid measurements, for which the use of multiple-pixel detectors, which are rather slow is not well suited. There is therefore a lack of a method that can guarantee a good measurement sensitivity for biological tissues.